Our invention concerns apparatus for data transfer between a transducer head and a dislike storage medium such as, typically, a flexible magnetic disk, now commonly known as a floppy disk. More specifically our invention pertains to such a disk file or drive incorporating a stepping motor of the four phase, single phase drive type to cause the incremental motion of the transducer head in a radial direction of the data storage medium in relation to the rotation of the latter.
Flexible magnetic disks have found widespread acceptance in formation processing and allied industries as compact data storage media. The magnetic disk in general use has a series of concentric magnetic tracks for direct accessing by a magnetic transducer head. Data transfer between disk and head takes place as the disk is rotated and the head moved radially of the disk. Generally employed for such radial movement of the transducer head with respect to the magnetic disk is a stepping motor, otherwise known as a stepper motor, magnetic stepping motor, step servo motor, etc. Dalziel et al. U.S. Pat. No. 3,678,481 and Castrodale et al. U.S. Pat. No. 4,089,029 respresent examples of disk files employing stepping motors.
It is essential in this type of disk files that the transducer head be positioned on the outermost track (track zero) of the disk for proper data transfer when the device is electrically turned on. The device has a provision (optical "track zero" sensor) for sensing the head position on the disk upon closure of the power switch. If it senses the head position on tracks other than track zero, the head is automatically returned to the required initial position, the writing or reading of information being initiated thereafter. In the use of a four phase, single phase drive stepping motor, however, there has been the possibility of the head position being determined as being on track zero while actually the head is not returned to the track zero position. This has lead to improper data transfer. We will discuss in the following paragraphs how this has occurred.
The four phase, single phase drive stepping motor has several (e.g. four) sets of first, second, third and fourth phase windings arranged sequentially on stator poles of annular arrangement around a rotor (see FIG. 3 of the drawings attached hereto). The stator windings correspond to respective tracks on the magnetic disk. Only the first phase windings are energized upon closure of the power switch. If then the rotor has already been aligned with the first phase winding of the first set which corresponds to track zero, it will be retained in that angular position, and a signal representative of the correct initial rotor position will be produced to allow the device to initiate data transfer. If the rotor has initially been aligned with the second phase winding of the first set, it will turn to the first phase winding upon excitation thereof. If the rotor has initially been aligned with the fourth phase winding of the first set, it will turn to the first phase winding of the second set adjoining the fourth phase winding of the first set and will subsequently be returned to the first phase winding of the first set.
The rotor has been held stationary, however, upon excitation of the first phase windings if it has initially been aligned with the third phase winding of the first set, for example. This is because the rotor is positioned at the point of magnetic equilibrium between the first phase windings of the first and second sets.
The rotor will nevertheless be subsequently returned to the first phase winding corresponding to track zero if the "track zero" sensor senses that the transducer head has not been returned thereto upon initial excitation of the first phase windings. However, the optical "track zero" sensor of current construction is prone to determine that the transducer head is on track zero when it actually is not. Generally the magnetic disk storage file employing a four phase, single phase drive stepping motor for head movement is conditioned to start data transfer when the following three requirements are all met:
1. The sensing of the track zero position of the transducer head by the optical "track zero" sensor from the position of a mechanical part in the radial head drive mechanism.
2. The excitation of the first phase windings of the stepping motor.
3. The excitation of the stepping motor windings in a sequence to cause radially outward motion of the transducer head with respect to the magnetic disk.
The disk file control system self satisfies the above second and third requirements upon closure of the power switch. It is the first requirement that determines whether the disk file can be properly conditioned for the start of data transfer or not. The conditioning of the device for the start of data transfer would be easy if the "track zero" sensor could accurately sense the head position on track zero. This is not so; operating optically, the "track zero" sensor output may indicate that the transducer head is on track zero when it is actually anywhere between track zero and track two.
Thus the foregoing three conditions may all be seemingly fulfilled even though the transducer head is actually on track two, with the stepping motor rotor detended in alignment with the third phase winding corresponding to track two. The result has been erroneous data transfer between magnetic disk and transducer head.